1. Field of the Invention
The present invention relates to the fabrication of semiconductor devices and more particularly to improved silicon-VLSI-compatible metal-semiconductor-metal (MSM) and lateral p-i-n (LPIN) photodetector structures and the method of making them.
2. Prior Art
Metal-semiconductor-metal photodetectors (MSM-PDs) are a class of photodetectors wherein light incident upon a semiconductor system produces photogenerated carriers, which carriers are collected via an electric field between a set of interdigitated Schottky contacts on the photoactive surface region of the semiconductor.
Lateral p-i-n photodetectors (LPIN-PDs) are a class of photodetectors wherein light incident upon a semiconductor system produces photogenerated carriers, which carriers are collected via an electric field between a set of lateral interdigitated p-n junctions on the photoactive surface region of the semiconductor.
More particularly, MSM photodetectors are constructed of interdigitated metal-semiconductor-metal electrodes that are disposed on the photoactive semiconductor material that is on the surface of the device. Their principle of operation is as follows. Upon the application of a bias voltage between the electrodes, radiation (i.e., photons) from an infra-red, visible, or UV source, impinging on the photoactive surface region of the semiconductor, will result in a current flow between adjacent electrodes. The current consists of electron-hole pairs that are generated when photons are absorbed in the semiconductor. These photogenerated charge carriers are driven and collected by the electric field that is formed by the voltage between the electrodes. A Schottky barrier at the junction or interface between each electrode and the photoactive semiconductor surface limits the current flow to that produced by the photons absorbed into the semiconductor. Photoactive materials that are most commonly used in MSM photodetectors are GaAs and GaInAs.
Among the attributes of MSM devices are their high sensitivity, very high speed of operation and relative ease of fabrication. MSM-PDs have been fabricated in a variety of semiconductor materials including silicon and the so-called III-V semiconductor compounds and their alloys. The III-V semiconductor materials are so called because they are based on elements found in column III and column V of the Periodic Table of Elements, e.g., Ga, Al, and In in column III; As, P, and Sb in column V. The most widely used III-V semiconductor materials for MSM-PDs are, as noted, GaAs and GaInAs.
The other above-noted form of photodetector involves the provision of alternate p+ and n+ implanted regions beneath and in ohmic contact with the interdigitated metal electrodes in such a manner as to create a set of lateral interdigitated p-n junctions. The resulting structure is called a lateral p-i-n photodetector (LPIN-PD). Depending on the polarity of the low doped surface region of the semiconductor ("intrinsic" layer), the p-n junctions are set between the p+ regions and the surface region (n-type semiconductor) or the n+ regions and the surface region (p-type semiconductor). In either case, the p-n junctions are operated under reverse bias in such a manner that i) the photoactive surface region between the adjacent p+ and n+ regions is completely depleted of free carriers and ii) a moderate to high electric field is established between the n+ and p+ interdigitated electrodes. Radiation (i.e., photons) from an infra-red, visible, or UV source, impinging on the photoactive surface region of the semiconductor, will result in a current flow between adjacent p+ and n+ electrodes. The current consists of electron-hole pairs that are generated when photons are absorbed in the semiconductor. These photogenerated charge carriers are driven and collected by the electric field that is formed by the voltage between the electrodes. Lateral p-i-n photodetectors have also been fabricated in GaAs and GaInAs, as described in the article:
S. TIWARI, J. BURROUGHES, M. S. MILSHTEIN, M. A. TISCHLER AND S. L. WRIGHT, "Lateral Ga.sub.0.47 In.sub.0.53 As and GaAs p-i-n photodetectors by self-aligned diffusion," IEEE Photonics Technology Letters, vol. 4, no. 4, (1992).
As between the two photodetectors, MSM devices are much more common than lateral p-i-ns, and the most common MSM devices are typically made in III-V semiconductor materials that are designed to operate in the infrared region of the electromagnetic spectrum, more specifically at wavelengths greater than 700 nm. Various examples of such devices involving the elements gallium and arsenic (Ga-As) are described in:
M. ITO, T. KUMAI, H. HAMAGUCHI, M. MAKIUCHI, K, NAKAI, O. WADA, and T. SAKURAI, "High-Speed Monolithically Integrated GaAs Photoreceiver using a Metal-Semiconductor-Metal Photodiode," Appl. Phys. Lett., vol. 47, no. 11, pp. 1129-1131, 1985.
D. L. ROGERS, "Monolithic Integration of a 3-GHz Detector/Preamplifier Using a Refractory-Gate, Ion-Implanted MESFET Process," IEEE Electron Device Lett., vol. EDL-7, no. 11, pp. 600-603, 1986.
T. SUGETA, T. URISU, S. SAKATA, AND Y. MIZUSHIMA, "Metal-Semiconductor-Metal Photodetector for High-Speed Optoelectronic Circuits," Japan. J. Appl. Phys., vol. 19, suppl. 19-1, pp. 459-464, 1980.
O. WADA, H. NOBUHARA, H. HAMAGUCHI, T. MIKAWA, A. TACKEUCHI, and T. FUJII, "Very High Speed GaInAs Metal-Semiconductor-Metal Photodiode Incorporating an AlInAs/GaInAs Graded Superlattice," Appl. Phys. Lett. vol: 54, no. 1, pp. 16-17, 1989.
D. L. ROGERS, J. M. WOODALL, G. D. PETTIT, AND D. MCINTURFF, "High-Speed 1.3-.mu.m GaInAs Detectors Fabricated on GaAs Substrates," IEEE Electron Device Lett., vol. 9, no. 10, pp. 515-517, 1988.
C. S. HARDER, B. VAN ZEGHBROECK, H. MEIER, W. PATRICK. and P. VETTIGER "5.2-GHz Bandwidth Monolithic GaAs Optoelectronic Receiver," IEEE Electron Device Lett., vol. 9, no. 4, pp. 171-173, 1988.
W. ROTH, H. SCHUMACHER, J. KLUGE, H. J. GEELEN, and H. BENEKING, "The DSI Diode--A Fast Large-Area Optoelectronic Detector," IEEE Trans. Electron Devices, vol. ED-32, no. 6, pp. 1034-1036, 1985.
M. ITO AND O. WADA, "Low Dark Current GaAs Metal-Semiconductor-Metal (MSM) Photodiodes Using WSi.sub.x Contacts," IEEE J. Quantum Electronic, vol QE-22, no. 7, pp. 1073-1077, 1986.
L. FIGUEROA AND W. SLAYMAN, "A Novel Heterostructure Interdigital Photodetector (HIP) with Picosecond Optical Response," IEEE Electron Device Lett., vol. EDL-2, no. 8, pp. 208-210, 1981.
On the other hand, short wavelength (&lt;850 nm) photodetectors are also important as they have potential widespread use in computing environments. For example, in optical storage systems, short wavelengths are preferred because recording density on optical disks can be increased. A further possible application of short wavelength detectors is in optical fiber interconnects to replace the expensive, bulky cables that are used with input/output (I/O) devices such as printers, monitors, and displays. For the latter application, the path lengths within the computer system are short, and consequently long wavelength detectors are not a critical requirement.
While all the MSM photodetectors cited above have been fabricated in the form of either discrete or integrated structures, they suffer from major drawbacks with regard to their integration into silicon-based technologies as presently practised in the computer industry. These drawbacks include high cost due to process complexity, and the use of materials and processes which are incompatibile with established silicon VLSI fabrication methods.
A particular drawback of GaAs MSM-PDs is the difficulty of incorporating them into silicon integrated circuits. A silicon-based photodetector technology could better fulfill the requirements for the above-noted computer environment applications with the key advantage of integrability with a mature CMOS or bipolar technology. This technology would render the incremental cost of the photodetectors minimal, and parasitic delays and costly assembly could be eliminated. Such MSM photodetectors lend themselves to easy integration and have the advantage of low capacitance combined with good responsivity. See, for example, the above-cited article of M. ITO ET AL, in Appl. Phys. Lett., vol. 47, no. 11, pp. 1129-1131, 1985.
Some silicon MSM photodetectors which operate in the ultraviolet region of the spectrum, more specifically at wavelengths below 400 nm, have been reported and described in the technical literature. Descriptions of MSM-PD's fabricated on bulk Si to avoid the problems with short wavelength applications are found, for example, in the recent article: B. W. MULLINS, S. F. SOARES, K. A. MCARDLE, C. M. WILSON AND S. R. J. BRUECK, "A Simple High-Speed Si Schottky Photodiode," IEEE Photonics Technol. Letts., vol., 3, no. 4, pp. 360-362, 1992.
Some earlier work in silicon-based MSM technology is described in the article:
R. J. SEYMOUR AND B. K. GARSIDE, "Ultrafast Silicon Interdigital Photodiodes for Ultraviolet Applications," Can. J. Phys., vol. 63, pp. 707-711, 1985.
In the former article, Schottky barriers are formed by disposing interdigitated metal fingers of Ni on bulk Si, while in the latter case the metal is Au on n-type Si on sapphire. However, the materials and processes employed in the fabrication of these devices are not compatible for purposes of integration with established silicon VLSI technology.
Consequently, it is an object of the present invention to provide a photodetector device that is compatible with silicon integrated circuits and suitable for short wavelength applications.
Another object of the present invention is to provide a photodetector with more efficient light sensitivity and speed of response.
Another object of the present invention is overcoming the problem in semiconductor photodetectors with interdigitated surface electrodes posed by longer wavelength light generating carriers deep within the semiconductor, beyond the influence of the electric field of the interdigitated surface electrodes and thus slow to be collected and slowing down the photoresponse of the detector.
Another object of the present invention is providing for the protection and passivation of the surface region of such photodetectors between the interdigitated electrodes, and extending the spectral sensitivity of the photoactive semiconductor surface region of the device.
A further object of the present invention is to reduce the electric field strength at the metal finger edges of MSM photodetectors, thus reducing the excessive leakage current due to tunneling and Schottky barrier-lowering phenomena.